Why did glacial cycles intensify a million years ago? Researchers find clues on the bed of the Atlantic Ocean

Why did glacier cycles intensify a million years ago?

A new study suggests that a million years ago glaciers began to stick to their beds more stubbornly, triggering cycles of longer ice ages. Here, ice is ejected from Iceland’s Breiðamerkurjökull glacier on its way into the Atlantic. Photo credit: Kevin Krajick / Earth Institute

Something great happened to the planet about a million years ago. There has been a major shift in the response of Earth’s climate system to changes in our orbit around the sun. The shift is known as the transition to the Middle Pleistocene. Before the MPT, cycles between the ice age (colder) and interglacial (warmer) took place every 41,000 years. After the MPT, the ice ages became more intense – intense enough to form ice sheets in the northern hemisphere that lasted 100,000 years. This gave the earth the regular ice age cycles that lasted into human time.

Scientists have long puzzled over what triggered this. One likely reason would be a phenomenon known as the Milankovitch Cycles – cyclical changes in Earth’s orbit and orientation with the Sun that affect the amount of energy the Earth absorbs. Scientists agree that this has been the most important natural driver for alternating warm and cold periods for millions of years. However, research has shown that the Milankovitch Cycles did not undergo major changes a million years ago, so something else was likely at work.

At the same time as the MPT, a large system of ocean currents that helps to transport heat around the globe experienced a severe weakening. This system that sends heat north through the Atlantic is the Atlantic Meridional Overturning Circulation (AMOC). Was this slowdown related to the shift in the ice ages? If so, how and why? Those were open questions. A new article was published in the journal today Proceedings of the National Academy of Sciences suggests an answer.

The researchers analyzed the cores of deep-sea sediments from the South and North Atlantic, where ancient deep water passed and left chemical traces. “We found that shortly before this crash, the North Atlantic behaved very differently from the rest of the basin,” said lead author Maayan Yehudai, who did the Ph.D. Student at Columbia University’s Lamont-Doherty Earth Observatory.

Before this crash in ocean circulation, the northern hemisphere’s ice sheets began to adhere more effectively to their bedrock. This resulted in the glaciers becoming thicker than before. This in turn resulted in more global cooling than before and disrupted the Atlantic heat conveyor belt. This has led to both stronger ice ages and a shift in the ice age cycle, says Yehudai.

Research supports a long-debated hypothesis that the gradual removal of accumulated slippery continental floors during earlier ice ages allowed the ice sheets to cling more tightly to the older, harder crystalline bedrock below and become thicker and more stable. The results suggest that this growth and stabilization shaped the global climate shortly before the AMOC weakened.

“Our research addresses one of the biggest questions about the biggest climate change since the Ice Ages,” said Yehudai. “It was one of the most significant climatic transitions, and we don’t fully understand it. Our discovery makes the origin of this change in the northern hemisphere and the ice sheets formed there as the impetus for this shift towards the climatic patterns we observe today. This is a very important step in understanding what caused it and where it is coming from. It underlines the importance of the North Atlantic region and the ocean circulation for current and future climate change. “

The research was also led by Yehudai’s advisor, Lamont geochemist Steven Goldstein, along with Lamont graduate student Joohee Kim. Other collaborators were Karla Knudson, Louise Bolge and Alberto Malinverno von Lamont-Doherty; Leo Pena and Maria Jaume-Segui from the University of Barcelona; and Torsten Bickert from the University of Bremen. Yehudai is now at the Max Planck Institute for Chemistry.

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More information:
Evidence for a trigger of the 100,000-year glacial cyclicity in the northern hemisphere, Proceedings of the National Academy of Sciences (2021). DOI: 10.1073 / pnas.2020260118.

Provided by the Earth Institute at Columbia University

Quote: Why did glacier cycles intensify a million years ago? Researchers find clues at the bottom of the Atlantic Ocean (2021, November 8), accessed November 8, 2021 from https://phys.org/news/2021-11-glacial-million-years-clues-bed.html

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